Direct measurements of length changes occuring in the fibers of the mammalian diaphragm under in situ conditions with the rib cage intact have not been previously reported. A small length gauge using the Hall effect to measure the distance between a transducer and a permanent magnet has been constructed in our laboratory. The topographical mechanical activity of the canine diaphragm can be monitored by suturing three of these devices to different abdominal regions of the muscle (right costal, left costal, crural) without a measurable effect on the chest wall pressure-volume relaxation characteristics. We propose to measure regional diaphragmatic lengths, rib cage and abdominal volumes, and esophageal and abdominal pressures during ventilation by passive lung inflation and by active contraction of the diaphragm. Theoretical models of the respiratory system will be compared to the experimental results in an attempt to discern whether the canine diaphragm operates as a piston in a cylinder or whether diaphragmatic radius changes influence the respiratory pressures (Laplace effect). The regional length measurements will be used to study the distribution of physiological loads on the diaphragm. The load will be experimentally altered to analyze the length-tension characteristics of the different areas of the muscle. A length-tension curve will be constructed using bilateral electrophrenic stimulation to attain isometric lengths at different system volumes for a given thoracoabdominal configuration. Transdiaphragmatic pressure will be used as an index of muscle tension. The velocity of shortening of contracting segments of the diaphragm will be compared to inspiratory airflow and abdominal volume flow. The force-velocity characteristics of the diaphragm will be diagramed using transdiaphragmatic pressure and the rate of shortening from the length gauges. Simultaneous regional length and electromyographic measurements will be used to assess the temporal sequence of physiological excitation and mechanical displacement of the diaphragm during normal and augmented breathing. Although these studies use the anesthetized dog as a model, we believe the results will have valid inferential application to man. Such studies may help clarify the action of the diaghragm in normal breathing and in various respiratory abnomalities such as emphysema.